For a rubber composition for tread of pneumatic tires, in particular a rubber composition for tread of high performance tires such as tires for competition, there is a strong demand of high grip performance from the beginning of running (initial grip performance) and stable grip performance during running (stability of grip performance), and various measures have been taken to secure the both performance.
A method of improving grip performance by compounding a resin having a particular physical property or chemical structure in a rubber composition for tread has been considered (JP 2004-137463 A and the like). For example, in order to improve initial grip performance, a method of increasing a compounded amount of a low softening point resin, liquid polymer and the like or a method of compounding a softener with a low temperature has been considered. On the other hand, in order to obtain stability of grip performance, a method of compounding a high softening point resin in a rubber composition for tread has been considered.
However, in tires having a tread compounding a low softening point resin, while initial grip performance is improved, there is a problem that stability of grip performance is lowered as a temperature of the tread increases. On the other hand, in tires having a tread compounding a high softening point resin, there is a problem that initial grip performance is significantly deteriorated while stability of grip performance can be obtained.
In order to solve these problems, a method of compounding a combination of a low softening point resin and a high softening point resin in a rubber composition is considered. However, since a total amount of resin compounded in a rubber composition widely affects a temperature property of the whole rubber, the total amount of resin which can be compounded into the rubber composition is limited. As a result, in a rubber composition using a combination of a low softening point resin and a high softening point resin, initial grip performance and stability of grip performance cannot be obtained as the case where each resin is compounded alone. Additionally, grip performance and abrasion resistance are counter-performance.
In view of a chemical structure of resins, a hydrocarbon resin containing a component unit derived from a monomer component having a heteroatom (for example, coumarone-indene resin, phenolic resin, which are hereinafter referred to as polar resins) has a high polarity and thus it adheres to a metal portion of a rotor or roller and arises a problem that processability thereof is deteriorated. Therefore, while these polar resins are excellent in stability of grip performance, the total amount of polar resins which can be compounded into a rubber composition is limited.
On the other hand, while a hydrocarbon resin that does not contain a component unit derived from a monomer component having a heteroatom (hereinafter referred to as nonpolar resins) is excellent in processability, abrasion resistance and initial grip performance, stability of grip performance is inferior compared to that of polar resins. It is noted that the reason of excellent processability, abrasion resistance and initial grip performance of nonpolar resins seems to be their non-polarity and high solubility with nonpolar rubber components.